Conventionally, there has been disclosed various howling cancellers capable of suppressing howling of a speech signal input from a microphone and emitting the resultant through a speaker and so forth.
A conventional howling canceller based on analog processing without using an adaptive filter has been disclosed in Patent literature 1. Patent literature 1 discloses a technology that assumes transmission characteristics of an acoustic system between a microphone of a hearing aid and a speaker are constant, and prevents the occurrence of howling by setting transmission characteristics of a feedback circuit to be equal to transmission characteristics of the acoustic system, which has been measured in advance, by using the feedback circuit with fixed characteristics. However, if a change occurs in the transmission characteristics of the acoustic system between the microphone and the speaker, it is difficult to suppress the howling with the technology disclosed in Patent literature 1.
In order to cope with the problem of the change in the transmission characteristics of the acoustic system, there has been disclosed a system capable of suppressing howling through digital processing using an adaptive filter in a loudspeaker. The system has a structure in which positive feedback is applied from the output to the input of an adaptive system having the same configuration as the system. In addition, a delay circuit is inserted into a feedback loop. The delay circuit improves convergence characteristics of the adaptive filter by reducing correlation between the output signal and the input signal of the adaptive system, which is caused by feedback.
If delay of the delay circuit is larger than an impulse response length of the system to be identified and disposed between the input of a D/A converter and the output of an A/D converter, and than an impulse response length of the adaptive filter, no increase of the correlation due to the feedback occur in principle.
In this system, even when howling occurs, if the adaptive filter can accurately estimate transmission characteristics between the input of the D/A converter and the output of the A/D converter, the howling can be suppressed.
However, when a gain of a loudspeaker system exceeds “1” in a wide frequency range, convergence of the adaptive filter does not catch up the rapid growth of the amplitude of howling sound caused by positive feedback, the amplitude of the howling sound increases beyond the linear region of any one of a D/A converter, a power amplifier, a speaker, a microphone, a microphone amplifier, and an A/D converter, and the waveform of the howling sound is saturated, resulting in the occurrence of non-linear distortion.
Since the adaptive filter performs a process based on the assumption of the linearity of a system, if non-linear distortion occurs in the progress of generating a desired signal to be estimated from an input signal, bias occurs in the operation of the adaptive filter and thus good convergence characteristics are not obtained. Therefore, if howling occurs in the loudspeaker system with a gain exceeding “1” over a wide frequency range and reaches a saturation state once, it is difficult to suppress the howling by the adaptive filter.
In order to solve the problem, Patent literature 2 discloses a technology capable of preventing saturation of an A/D converter and a D/A converter by using a limiter circuit in an active noise canceller using an adaptive filter.
Furthermore, Patent literature 3 discloses a technology capable of correcting and removing non-linear distortion by using the Volterra filter in order to prevent an adverse influence of the non-linear distortion occurring in a speaker, on the convergence characteristics of a howling canceller.
Furthermore, Patent literature 4 discloses a technology capable of achieving an effect similar to a change in transmission characteristics through a conversion process of a non-linear signal, and suppressing the rapid growth of howling.